Frankfurt 2006 – scientific programme

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MO: Molekülphysik

MO 71: Dynamics and Spectroscopy at Interfaces

MO 71.3: Talk

Friday, March 17, 2006, 11:10–11:25, H12

Observation of pH dependent back-electron transfer dynamics in Alizarin - TiO2 adsorbates: Importance of trap states — •Matylitsky Victor , Lenz Martin, and Wachtveitl Josef — Goethe -University Frankfurt/Main, Institute of Physical and Theoretical Chemistry, Marie-Curie-Str. 11, D-60439 Frankfurt/Main, Germany

The dependence of the interfacial electron transfer in alizarin sensitized TiO2 nanoparticles from sample pH has been examined via transient absorbance spectroscopy in the visible spectral region. Excitation of the alizarin/TiO2 system leads to a very fast electron injection (<100 fs) over a wide pH range. Back electron transfer shows a complicated multiphasic kinetics and strongly depends on the acidity of the solution. The drastic changes in electron transfer dynamics upon varying the pH are explained by Nernstian-type change in the semiconductor band energy. Indeed, a change of sample pH values over seven units leads to a ~0.42 eV change of the conduction band edge position (i.e. the nominal free energy of the electron in the electrode). This change was sufficient to push the system to a condition, where direct electron injection to intraband gap surface states could be investigated. The existence of an electron transfer pathway via surface trap states is supported by the similarity of the observed back electron transfer kinetics of alizarin/TiO2 at pH 9 and alizarin/ZrO2 reported in earlier work (J. Phys. Chem. B 2000, 104, 8995), where the conduction band edge is approximately 1eV above the excited state of the dye. The influence of surface trap states on interfacial electron transfer and a detailed analysis of their population, depopulation and relaxation kinetics will be presented.